Microstructure array and a microlens array

ABSTRACT

In a method of fabricating an array of microstructures, a substrate with an electrically-conductive portion is provided, an insulating mask layer is formed on the electrically-conductive portion of the substrate, a plurality of openings are formed in the insulating mask layer to expose the electrically-conductive portion, and a first plated or electrodeposited layer is deposited in the openings and on the insulating mask layer by electroplating or electrodeposition. A second plated layer is further formed on the first plated or electrodeposited layer and on the electrically-conductive portion by electroless plating to reduce a size distribution of microstructures over the array.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of Application No. 10/449,103, filed Jun.2, 2003, now U.S. Pat. No. 6,835,443 which in turn, is a division ofapplication Ser. No. 09/534,070, Mar. 24, 2000, now U.S. Pat. No.6,632,342 B1. Both of these prior applications are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for fabricating amicrostructure array, a method for fabricating a mold or a master of amold (in the specification the term “mold” is chiefly used in a broadsense including both a mold and a master of a mold) for forming amicrostructure array, a method for fabricating a microstructure arrayusing the mold, and a microstructure array. This invention particularlyrelates to a mold for forming a microlens array, a method forfabricating the mold, and a method for fabricating the microlens arrayusing the mold.

2. Description of the Related Background Art

A microlens array typically has a structure of arrayed minute lenseseach having a diameter from about 2 or 3 microns to about 200 or 300microns and an approximately semispherical profile. The microlens arrayis usable in a variety of applications, such as liquid-crystal displaydevices, optical receivers and inter-fiber connections in opticalcommunication systems.

Meanwhile, earnest developments have been made with respect to a surfaceemitting laser and the like which can be readily arranged in an arrayform at narrow pitches between the devices. Accordingly, there exists asignificant need for a microlens array with narrow lens intervals and alarge numerical aperture (NA).

Likewise, a light receiving device, such as a charge coupled device(CCD), has been repeatedly decreased in size as semiconductor processingtechniques have been developed and advanced. Therefore, also in thisfield, the need for a microlens array with narrow lens intervals and alarge NA is increasing.

In the field of such a microlens, a desirable structure is a microlenswith a large light-condensing efficiency which can highly efficientlyutilize light incident on its lens surface.

Further, similar desires exist in the fields of optical informationprocessing, such as optical parallel processing-operations, and opticalinterconnections. Furthermore, active or self-radiating type displaydevices, such as electroluminescence (EL) panels, have beenenthusiastically studied and developed, and a highly-defined andhighly-luminous display has been proposed. In such a display, there is aheightened desire for a microlens array which can be produced at arelatively low cost and with a large area, as well as with a small lenssize and a large NA.

There are presently a number of prior art methods for fabricatingmicrolenses.

In a prior art microlens-array fabrication method using an ion exchangemethod (see M. Oikawa, et al., Jpn. J. Appl. Phys. 20 (1) L51-54, 1981),a refractive index is raised at plural places in a substrate ofmulti-component glass. A plurality of lenses are thus formed places witha high-refractive index. In this method, however, the lens diametercannot be large, compared with the intervals between lenses. Hence it isdifficult to design a lens with a large NA. Further, the fabrication ofa large-area microlens array is not easy since a large scalemanufacturing apparatus, such as an ion diffusion apparatus, is requiredto produce such a microlens array. Moreover, an ion exchange process isneeded for each glass, in contrast with a molding method using a mold.Therefore, variations of lens quality, such as a focal length, arelikely to increase between lots unless the management of fabricationconditions in the manufacturing apparatus is carefully conducted. Inaddition, to the above, the cost of this method is relatively high, ascompared with the method using a mold.

Further, in the ion exchange method, alkaline ions for ion-exchange areindispensable in a glass substrate, and therefore, the material of thesubstrate is limited to alkaline glass. The alkaline glass is, however,unfit for a semiconductor-based device which needs to be free ofalkaline ions. Furthermore, since a thermal expansion coefficient of theglass substrate greatly differs from that of a substrate of a lightradiating or receiving device, misalignment between the microlens arrayand the devices is likely to occur due to a misfit between their thermalexpansion coefficients as an integration density of the devicesincreases.

Moreover, a compressive strain inherently remains on the glass surfacewhich is processed by the ion exchange method. Accordingly, the glasstends to warp, and hence, a difficulty in joining or bonding between theglass and the light radiating or receiving device increases as the sizeof the microlens array increases.

In another prior art microlens-array fabrication method using a resistreflow (or melting) method (see D. Daly, et al., Proc. Microlens ArraysTeddington., p23-34, 1991), resin formed on a substrate is cylindricallypatterned using a photolithography process and a microlens array isfabricated by heating and reflowing the resin. Lenses having variousshapes can be fabricated at a low cost by this resist reflow method.Further, this method has no problems of thermal expansion coefficient,warp and so forth, in contrast with the ion exchange method.

In the resist reflow method, however, the profile of the microlens isstrongly dependent on the thickness of resin, wetting conditions betweenthe substrate and resin, and the heating temperature. Therefore,variations between lots are likely to occur while fabricationreproducibility per a single substrate surface is high.

Further, when adjacent lenses are brought into contact with each otherdue to the reflow, a desired lens profile cannot be secured due to thesurface tension. Accordingly, it is difficult to achieve a highlight-condensing efficiency by bringing the adjacent lenses into contactand decreasing an unused area between the lenses. Furthermore, when alens diameter from about 20 or 30 microns to about 200 or 300 microns isdesired, the thickness of deposited resin must be large enough to obtaina spherical surface by the reflow. It is, however, difficult touniformly and thickly deposit the resin material having desired opticalcharacteristics (such as refractive index and optical transmissivity).Thus, it is difficult to produce a microlens with a large curvature anda relatively large diameter.

In another prior art method, an original plate of a microlens isfabricated, lens material is deposited on the original plate and thedeposited lens material is then separated. The original plate or mold isfabricated by an electron-beam lithography method (see Japanese PatentApplication Laid-Open No. 1 (1989)-261601), or a wet etching method (seeJapanese Patent Application Laid-Open No. 5 (1993)-303009). In thesemethods, the microlens can be reproduced by molding, variations betweenlots are unlikely to occur, and the microlens can be fabricated at a lowcost. Further, the problems of alignment error and warp due to thedifference in the thermal expansion coefficient can be solved, incontrast with the ion exchange method.

In the electron-beam lithography method, however, an electron-beamlithographic apparatus is expensive and a large investment in equipmentis needed. Further, it is difficult to fabricate a mold having a largearea more than 100 cm² (100 cm-square) because the electron beam impactarea is limited.

Further, in the wet etching method, since an isotropic etching using achemical action is principally employed, an etching of the metal plateinto a desired profile cannot be achieved if the composition andcrystalline structure of the metal plate vary even slightly. Inaddition, etching will continue unless the plate is washed immediatelyafter a desired shape is obtained. When a minute microlens is to beformed, a deviation of the shape from the desired one is possible due toetching lasting during a period from the time a desired profile isreached to the time the microlens is reached.

Further, there also exists a mold fabrication method using anelectroplating technique (see Japanese Patent Application Laid-Open No.6 (1994)-27302). In this method, an insulating film having a conductivelayer formed on one surface thereof and an opening is used, theelectroplating is performed with the conductive layer acting as acathode, and a protruding portion acting as a mother mold for a lens isformed on a surface of the insulating film. The process of fabricatingthe mold by this method is simple, and cost is reduced. Similar suchmethods are also disclosed in Japanese Patent Application Laid-Open No.8 (1996)-258051 and Japanese Patent Publication No. 64 (1989)-10169.

The problem occurring when a plated layer is formed in an opening by theelectroplating technique will be described by reference to FIGS. 1A and1B. FIGS. 1A and 1B illustrate a radius variation or distribution ofplated layers 105 formed in a two-dimensional array on a substrate 101.In the above fabrication method using electroplating in anelectroplating bath, a distribution or variation of anelecroplating-current density occurs over the substrate 101 due to apattern of the openings (i.e., the electrode pattern) formed in aninsulating mask layer 103 to expose an electrode layer 102. Morespecifically, the electric field is unevenly concentrated (stronger in aperipheral region than in a central region), and the electroplatinggrowth is hence promoted near the periphery of the pattern of thearrayed openings. As a result, there is a distribution or variation ofthe size of semispherical microstructures 105 on the substrate.Therefore, when this substrate is used as a mold for forming a microlensarray, the specifications of respective microlenses vary over the array.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method forfabricating a microstructure array (typically a microlens array such asa semispherical microlens array, a flyeye lens and a lenticular lens)with good performance and a reduced size distribution of microstructuresflexibly, readily and stably; a method for fabricating a mold forforming a microstructure array; a fabrication method for amicrostructure array using the mold, and so forth.

The present invention is generally directed to a method for fabricatingan array of microstructures which includes the following steps:

preparing a substrate with an electrically-conductive portion;

forming an insulating mask layer on the electrically-conductive portion;

forming a plurality of openings in the insulating mask layer to exposethe electrically-conductive portion;

forming a first plated or electrodeposited layer in the opening and onthe insulating mask layer by electroplating or electrodeposition; and

forming a second plated layer on the first plated or electrodepositedlayer and on the electrically-conductive portion by electroless platingto reduce a size distribution of microstructures over the array.

In the above fabrication method, the first plated layer is formed byelectroplating, or the first electrodeposited layer is formed byelectrodeposition using an electrodepositable organic compound. Thearray pattern is typically a two-dimensional array pattern which isperiodical in at least a direction, a two-dimensional array patternwhich is periodical in four mutually-orthogonal directions, or aperiodical stripe pattern. In terms of an influence of the pattern ofthe conductive portion exposed through the openings-on the distributionof a current density at the time of the electroplating orelectrodeposition, the array pattern may create a current distributionin which the current density is uneven over the array. Typically, theopening has a circular shape and the microstructure is a semisphericalmicrostructure, or the opening has an elongated stripe shape and themicrostructure is a semicylindrical microstructure.

More specifically, the following constructions are possible based on theabove fundamental construction.

The second plated layer is formed by electroless plating using anelectroless plating solution containing a reducing agent ofhypophosphite such as sodium hypophosphite. Thereby, corrosionresistance and wear resistance of the microstructure array can beimproved.

The fabrication method may further include a step of forming a thirdplated layer on the second plated layer by electroplating, or a step offorming a third plated layer on the second plated layer by electrolessplating which preferably uses an electroless plating solution containinga reducing agent of hypophosphite. Thereby, corrosion resistance andwear resistance of the microstructure array can be improved.

The third plated layers may be continuously formed at their bottomportions. A flyeye lens can be fabricated by using the microstructurearray fabricated by such a method.

In the fabrication method, the first plated or. electrodeposited layerand the second plated layer (additionally, the third plated layer) maybe formed such that a horizontal bottom diameter or width of asemispherical or semicylindrical microstructure is approximately in arange from 1 μm to 200 μm. Such a minute microlens array is stronglydesired with accurate size, good controllability and high stability, andthe fabrication method of this invention can meet this desire.

In the fabrication method, the first plated or electrodeposited layerand the second plated layer (additionally, the third plated layer) maybe formed such that a distribution of horizontal bottom diameters orwidths of semispherical or semicylindrical microstructures (in thisspecification, the distribution is used as a ratio of a differencebetween a maximum value and a minimum value relative to the minimumvalue concerning the size of microstructures) is approximately less than20%. When the size distribution takes such a value, the microstructurearray, such as a mold for forming a microlens array, is of practicaluse.

In the fabrication method, the first plated or electrodeposited layermay be formed such that a ratio of a horizontal bottom diameter or widthof the first plated or electrodeposited layer relative to a horizontalbottom diameter or width of a semispherical or semicylindricalmicrostructures is approximately less than 0.5. When such a condition issatisfied, a satisfactory size distribution can be readily achieved. Asthe thickness of the electroless plated layer in a vertical directionincreases relative to the entire thickness or radius in the verticaldirection of the microstructure, the size distribution of themicrostructures decreases. Therefore, in order to better achieve a smalldistribution, a thickness ratio of the electroless plated layer relativeto the entire microstructure is preferably as large as possible. On theother hand, the process speed of the electroless plating is lower thanthat of the electroplating or electrodeposition. The above ratio isdetermined considering the above factors.

In the fabrication method, the first plated or electrodeposited layermay be formed such that a diameter or width of the first plated orelectrodeposited layer is approximately less than 10 μm. Thereby, themicrostructure array with microstructures having a bottom diameter orwidth approximately in a range from 1 μm to 200 μm can be readilyachieved with a preferable distribution.

The fabrication method can further include a step of forming a mold onthe substrate with the first plated or electrodeposited layer and thesecond plated layer (additionally, the third plated layer) by, forexample, electroplating, and a step of separating the mold from thesubstrate. Thereby, a mold for forming a microstructure array such as amicrolens array can be fabricated.

The fabrication method can further include a step of coating thesubstrate having the first plated or electrodeposited layer and thesecond plated layer (additionally, the third plated layer) with a firstresin, a step of hardening the first resin, a step of separating thefirst resin from the substrate, and a step of coating the hardened firstresin with a second resin having a refractive index different from arefractive index of the first resin. Thereby, a preferable microlensarray can be fabricated.

The present invention is also directed to a microstructure arrayincluding the following:

a substrate having an electrically-conductive portion;

an insulating mask layer formed on the electrically-conductive portion,in which a plurality of openings are formed to expose theelectrically-conductive portion;

a first plated or electrodeposited layer formed in the opening and onthe insulating mask layer by electroplating or electrodeposition; and

a second plated layer formed on the first plated or electrodepositedlayer and on the electrically-conductive portion by electroless plating.

Also in this microstructure array, the above specific structures may beadopted. The microstructure array is typically a mold for forming amicrolens array, a lenticular lens or a flyeye lens.

These advantages and others will be more readily understood inconnection with the following detailed description of the more preferredembodiments in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a cross-sectional view and a plan view illustratinga conventional microstructure array formed on a substrate, respectively.

FIGS. 2A to 2D are cross-sectional views illustrating fabrication stepsof a method for fabricating a microlens array mold or the like of firstand second embodiments according to the present invention, respectively.

FIG. 3 is a view illustrating an electroplating apparatus used in thepresent invention.

FIG. 4 is a view illustrating an electroless plating apparatus used inthe present invention.

FIG. 5 is a view illustrating a principle of forming a semispherical orsemicylindrical microstructure by electroplating or electrodepositionused in the present invention.

FIGS. 6A to 6E are cross-sectional views illustrating fabrication stepsof a fabrication method of fabricating a microlens array mold or thelike of third and fourth embodiments according to the present invention,respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

A first embodiment of a method for fabricating a semisphericalmicrostructure array will be described by reference to FIGS. 2A to 2D.

Initially, a silicon wafer of 1 inch in diameter is thermally oxidizedusing an oxidizing gas, and layers of silicon dioxide with a thicknessof 1 μm are formed on opposite surfaces of the wafer. This wafer is usedas a substrate 1 illustrated in FIGS. 2A to 2D. Cr and Au arecontinuously layered with thicknesses of 10 nm and 200 nm on the abovewafer, respectively, using an electron beam vacuum-evaporation methodwhich is a suitable thin-film forming method. An electrode layer 2 isthus formed.

A photoresist is then deposited as an insulating mask layer 3 asillustrated in FIG. 2A. Openings 4 are then formed in the mask layer 3by photolithography using exposure and development. A plurality of theopenings 4 are thus formed in a two-dimensional matrix array of 700×700to expose the electrode layer 2 as illustrated in FIG. 2B. The opening 4has a circular shape and a diameter of 5 μm. Intervals between theadjacent openings 4 are 25 μm.

Ni (nickel) electroplating is then performed for sixty (60) seconds at abath temperature of 500° C. and a cathodic current density of 40 A/dm².The above substrate 1 for electroplating is used as a base 7, and theelectrode layer 2 is used as a cathode as illustrated in FIG. 3. Nielectroplating bath 20 containing nickel (II) sulfate, nickel (II)chloride, boric acid and brightener is used. Ni plated layer 5 isinitially deposited in the opening 4 and grown therein. The plated layer5 expands onto the musk layer 3. The semispherical or semicylindricalplated layer 5 is deposited until a diameter of its bottom portion 6 areaches 11 μm in a central portion of the array, as illustrated in FIG.2C. In FIG. 3, an external electric power source 9 is connected betweenthe base 7 and an anodic place 8.

Ni electroless plating is then performed at a bath temperature of 90° C.to form electroless plated layers 6 as illustrated in FIG. 2D. Nielectroless plating solution (S-780 (trade name) produced by NihonKanizen Com.) containing a reducing agent of hypophosphite is used.

The Ni plated layer 6 obtained by the electroless plating containsphosphorus. When measured, a diameter of a bottom portion of the platedlayers 5 and 6 in the central portion of the array was 21 μm and adiameter ratio of the electroplated layer 5 relative to the platedlayers 5 and 6 was 0.52. At this time, a diameter of a bottom portion ofthe plated layers 5 and 6 in a peripheral portion of the array was 27 μm(the maximum value). A distribution of diameters of bottoms of theplated layers in central and peripheral portions of the array (i.e., aratio of a difference between maximum and minimum diameters relative tothe minimum diameter) was about 28%.

For comparison with the above, the above Ni electroplating was conductedfrom beginning to end until the diameter of a bottom portion of a platedlayer in a central portion of the array reached 21 μm. When diameters ofbottoms of the plated layers in several regions of the array weremeasured, the maximum diameter was found to be 33 μm in the peripheralportion of the array and the distribution of diameters of bottoms of theplated layers was found to be about 55%. It can be seen therefrom that amold for forming a microlens array with a reduced diameter distribution(reduced to 28% from 55%) can be fabricated in this embodiment.

As described in the foregoing, in the mold of this embodiment, thedistribution or variation of the semispherical or semicylindrical platedlayers is decreased by the formation of the electroless plated layer onthe electroplated layer. Further, since the electroless plated layer 6contains phosphorus, corrosion resistance and wear resistance of themold are improved as compared with those of a mold produced by usingonly electrodeposition or electroplating.

Any material, such as metal, semiconductor (a silicon wafer or the like)and insulating substance (such as glass, quartz and polymer film), canbe used as the substrate material. When the metal material is used asthe substrate 1, there is no need to form the electrode layer 2.Further, when the semiconductor is used, the electrode layer is notnecessarily needed if the semiconductor has enough conductivity toenable electroplating. However, where metal or semiconductor is used asthe substrate, a plated layer will also be formed on a portion otherthan the microstructure forming portion, since the entire substrate isimmersed in the electroplating solution. Therefore, when the platedlayer is to be formed only on a predetermined portion, the insulatingsubstance can be preferably used as the substrate. Alternatively, ametal or semiconductor, whose surface is partially insulated, may alsobe used.

Materials of the electrode layer and the substrate are selected frommaterials which are not corrosive to the electro- or electroless-platingor electrodepositing solution used since the electrode layer is exposedto such a liquid. The mask layer 3 may be formed of any inorganic ororganic insulating material that is also anticorrosive to the electro-or electroless-plating or electrodepositing solution. Further, it ispreferable that it is harder to deposit the electroless plated layer onthe material of the mask layer 3 than on the previously-formed plated orelectrodeposited layer when electroless plating is conducted. Such amaterial is suitable for the mask layer 3. The material of the masklayer 3 is also anticorrosive to the electro- or electroless-plating orelectrodepositing solution.

Where the electroplating or electrodeposition is effected at the opening4 in the electroplating or electrodeposition solution 20 containing ionssuch as metal ions, ions in the electroplating or electrodepositionsolution 20 move toward the plated layer 2, and hence, deposition of theelectroplating or electrodeposition proceeds with its growth directionbeing isotropic, as illustrated in FIG. 5. Thus, a semispherical orsemicylindrical layer can be formed. When the size of the opening 4 issufficiently smaller than the size of the anodic plate 8 and ions areuniformly dissolved in the electroplating or electrodeposition solution20, the growth direction of the plated layer is isotropic. Typically, amicrolens array has a structure of arrayed minute lenses each having adiameter from about 2 or 3 microns to about 200 or 300 microns, and thesize of the opening 4 is made smaller than the desired diameter of themicrolens. In order to better achieve an isotropic growth of the platedor electrodeposited layer, the size of the opening is less than thediameter of the semispherical structure.

In the case of electroplating, the plated layer is formed by thedeposition of metal ions in the electroplating bath caused by theelectrochemical reaction. The thickness of the plated layer can bereadily controlled by controlling the electroplating time andtemperature. The following materials can be used as electroplatingmetal. For example, as a single metal, Ni, Au, Pt, Cr, Cu, Ag, Zn andthe like can be employed. As an alloy, Cu—Zn, Sn—Co, Ni—Fe, Zn—Ni andthe like can be used. Any material can be used so long as electroplatingis possible.

As the electrodeposition substance electrodeposited using a current andthe base 7 in a conventional electrodeposition apparatus, there can beemployed electrodepositable organic compound (acryl-series carboxylicacid resin and the like in the case of the anionic-typeelectrodeposition, and epoxy-series resin and the like in the case ofthe cationic-type electrodeposition).

FIG. 4 illustrates an electroless plating apparatus. The electrolessplated layer 6 is grown on the semispherical or semicylindrical platedor electrodeposited layer 5 until a desired radius is reached. Thediameter distribution of final microstructures results from the platedor electrodeposited layer, but not from the electroless plated layer.The deposition mechanism of the electroless plated layer is due to anoxidation-reduction reaction of metallic salt that requires no currentfor deposition of the plated layer. The electroless plated layers 6uniformly grow over the entire array. Electroless plating is stoppedwhen the substrate is taken out of electroless plating liquid 30 andwashed by water after the desired profile is achieved.

In the case of electroless plating, the thickness of the plated layer 6can be readily controlled by controlling electroless plating time andtemperature. The following materials can be used as electroless platingmetal, for example. As a single metal, Ni, Au, Cu, Co and the like canbe employed. As an alloy, Co—Fe, Co—W, Ni—Co, Ni—Fe, Ni—W and the likecan be used. Any material can be used so long as electroless plating ispossible.

As the reducing agent, sodium hypophosphite, potassium hypophosphite,sodium borohydride, potassium borohydride, hydrazine, formalin, tartaricacid and the like can be employed. When sodium hypophosphite orpotassium hypophosphite is used as the reducing agent, the electrolessplated layer contains phosphorus. Hence, corrosion resistance and wearresistance of the plated layer can be improved.

As discussed above, profiles of plated or electrodeposited layers can bereadily controlled by controlling processing time and temperature, sothe method has excellent controllability. When the electroless platingafter the initial electroplating or electrodeposition is stoppedimmediately before the desired profile is reached and the plated layerof electroplating material with high corrosion resistance and hardnessis then grown until the desired profile is achieved, a microstructurearray such as a mold for a microlens array with high corrosionresistance and hardness can be obtained.

A process of forming a microlens array by using the above structure willbe described. A resin of ultraviolet-ray hardening photopolymer isdeposited on a mold for a microstructure array obtained by the abovefabrication method. After a support substrate of glass is placed on theresin, the resin is hardened by exposing the resin to ultraviolet rays.The resin of a microlens array can be separated from the substrate withthe microstructure array by lifting the glass substrate. Thus, a concaveresin of the microstructure array can be formed.

Another resin with a larger refractive index than that of the aboveresin is further dropped on the concave resin, and the resin ishardened. Thus, a plane microlens array can be obtained.

In the above method, alkaline glass is not indispensable for forming amicrolens, so that materials of the microlens and the substrate are lessrestricted than in the ion exchange method.

The above microlens array may be fabricated by other methods, such as amethod in which a conventional thermoplastic resin is used and a heatedmold is stamped on this resin; a method in which a thermosetting resinis laid over a mold and then heated to be hardened; and a method inwhich an electron-beam hardening resin is coated on a mold and the resinis hardened by electron beam irradiation.

A fabrication process of forming a mold for forming a microlens array byusing the above structure will be described.

This fabrication method may further include a step of forming a mold onthe substrate obtained by the above fabrication method, and a step ofseparating the mold from the substrate. In this case, the mold may beformed using electroplating. Then, a convex microlens array isfabricated by molding using the mold.

In this fabrication method, a plurality of molds with the same profilecan be readily fabricated since the mold is formed by molding. In thisembodiment, a plurality of molds for a microlens array with the sameprofile can be fabricated using the same mold master.

After ultraviolet-ray hardening resin of photopolymer is laid over themold for a convex microlens array fabricated by the above method, aglass substrate as a supporting substrate is then placed on the resin.The resin is exposed to ultraviolet rays through the glass to behardened. After that, the glass with the resin is separated from themold. Thus, a convex microlens array is obtained. A plurality ofmicrolens arrays of photopolymer could be formed by repeating the samesteps using the same mold.

The above microlens array may also be fabricated by other methods, suchas the above-described methods using thermoplastic resin, thermosettingresin and electron-beam hardening resin.

In this method, the mold can be directly formed by electroplating or thelike. Therefore, no expensive equipment is needed, costs can be reduced,and the size of the mold can be enlarged readily. Furthermore, the sizeof the plated layer can be controlled in situ, and the lens diameter andthe like can be readily and precisely controlled by controllingprocessing time and temperature.

Second Embodiment

A second embodiment of a fabrication method of a semisphericalmicrostructure array will be described by reference to FIGS. 2A to 2D asthe first embodiment is described.

Initially, electrode layer 2, mask layer 3 and openings 4 are formed ona substrate 1 as in the first embodiment.

Ni (nickel) electroplating is then performed for ten (10) seconds at abath temperature of 50° C. and a cathodic current density of 40 A/dm².The above substrate 1 for electroplating is used as a base 7, and theelectrode layer 2 is used as the cathode as illustrated in FIG. 3. Nielectroplating bath containing nickel (II) sulfate, nickel (II)chloride, boric acid and brightener is used. Thus, semispherical orsemicylindrical plated layers 5 are deposited until a diameter of itsbottom portion reaches 6 μm in a central portion of the array, asillustrated in FIG. 2C.

Ni electroless plating is then performed at a bath temperature of 90° C.to form electroless plated layers 6 as illustrated in FIG. 2D. Nielectroless plating solution (S-780) containing a reducing agent ofhypophosphite is used.

The Ni plated layer 6 obtained by the electroless plating containsphosphorus. When measured, a diameter of a bottom portion of the platedlayers 5 and 6 in the central portion of the array was 21 μm and adiameter of a bottom portion of the plated layers 5 and 6 in aperipheral portion of the array was 23 μm (the maximum value). Thedistribution of diameters of bottoms of the plated layers in central andperipheral portions of the array was about 10%. The diameterdistribution of the second embodiment is smaller than that of the firstembodiment because a ratio of the electroplated layer 5 to the platedlayers 5 and 6 is reduced in the second embodiment.

Also in the mold of this embodiment, the distribution or variation ofthe semispherical or semicylindrical plated layers is decreased by theformation of the electroless plated layer on the electroplated layer.Further, since the electroless plated layer 6 contains phosphorus,corrosion resistance and wear resistance of the mold are improved ascompared with those of a mold produced by using only electrodepositionor electroplating.

Third Embodiment

A third embodiment of a method for fabricating a semisphericalmicrostructure array will be described by reference to FIGS. 6A to 6E.

Initially, electrode layer 11, mask layer 12 and openings 13 are formedon a substrate 10 as illustrated in FIGS. 6A and 6B similarly to thefirst embodiment.

A Cu (copper) electroplating is then performed for two (2) minutes at abath temperature of 55° C. and a cathodic current density of 4 A/dm² asillustrated in FIG. 3. The above substrate 10 for electroplating is usedas a base 7, and the electrode layer 11 is used as the cathode. A Cuelectroplating bath containing copper sulfate, sulfuric acid,hydrochloric acid and brightener is used. A Cu plated layer 14 isinitially deposited in the opening 13 and grown therein. The platedlayer 14 expands onto the mask insulating layer 12 as illustrated inFIG. 6C. The plated layer 14 is deposited until a diameter of its bottomportion reaches 6 μm in a central portion of the array.

A Au (gold) electroless plating is then performed at a bath temperatureof 93° C. to form electroless plated layers 15 as illustrated in FIG.6D. A Au (gold) electroless plating solution containing potassium goldcyanide, ammonium chloride, sodium citrate and sodium hypophosphite isused.

The Au plated layers 15 obtained by the electroless plating containsphosphorus. The electroless plated layer 15 is deposited until adiameter of its bottom portion reaches 15 μm in the central portion ofthe array.

A Cr (chromium) electroplating is then performed for sixty (60) secondsat a bath temperature of 50° C. and a cathodic current density of 4A/dm² to form a plated layer 16 and hence improve the corrosionresistance of the plated layer, as illustrated in FIG. 6E. The aboveelectroless plated layer 15 is used as the cathode. A Cr electroplatingbath containing chromic acid and sulfuric acid is used. Thus, the Cuplated layer 14 formed by electroplating, the Au electroless platedlayer 15 containing phosphorus, and the Cr plated layer 16 formed byelectroplating are deposited on the electrode layer 11 in this order.

When measured, a diameter of a bottom portion of the plated layers 14,15 and 16 in the central portion of the array was 20 μm and a diameterof a bottom portion of the plated layers 14, 15 and 16 in a peripheralportion of the array was 24 μm (the maximum value). The distribution ofthe diameters of the bottoms of the plated layers in central andperipheral portions of the array was about 20%.

Also in the mold of this embodiment, the distribution or variation ofthe semispherical or semicylindrical plated layers is decreased by theformation of the electroless plated layer on the electroplated layer.Further, since the chromium plated layer 16 is formed on the surface,hardness of the mold is improved.

Fourth Embodiment

A fourth embodiment of a method of fabricating a semisphericalmicrostructure array will be described by reference to FIGS. 6A to 6E.

Initially, electrode layer 11, mask layer 12 and openings 13 are formedon a substrate 10 as illustrated in FIGS. 6A and 6B, similarly to thethird embodiment.

Ni electroplating is then performed for ten (10) seconds at a bathtemperature of 50° C. and a cathodic current density of 40 A/dm². Theabove substrate 10 for electroplating is used as a base 7, and theelectrode layer 11 is used as the cathode as illustrated in FIG. 3. Nielectroplating bath containing nickel (II) sulfate, nickel (II)chloride, boric acid and brightener is used. Thus, semispherical orsemicylindrical plated layers 14 are deposited until a diameter of itsbottom portion reaches 6 μm in a central portion of the array, asillustrated in FIG. 6C.

Ni electroless plating is then performed at a bath temperature of 90° C.to form electroless plated layers 15 as illustrated in FIG. 6D. Nielectroless plating solution (S-780) containing a reducing agent ofhypophosphite is used. The Ni plated layers 15 obtained by theelectroless plating contain phosphorus. The plated layer 15 is depositeduntil a diameter of its bottom portion reaches 21 μm in a centralportion of the array.

An Au (gold) electroless plating is then performed for two (2) minutesat a bath temperature of 93° C. to form electroless plated layers 16 andhence improve the corrosion resistance of the plated layer, asillustrated in FIG. 6E. The above electroless plated layer 15 is used asthe cathode. An Au (gold) electroless plating solution containingpotassium gold cyanide, ammonium chloride, sodium citrate and sodiumhyphosphite is used.

Thus, the Ni plated layer 14 formed by electroplating, the Nielectroless plated layer 15 containing phosphorus, and the Auelectroless plated layer 16 containing phosphorus are deposited on theelectrode layer 11 in this order.

When measured, a diameter of a bottom portion of the plated layers 14,15 and 16 in the central portion of the array was 21 μm and a diameterof a bottom portion of the plated layers 14, 15 and 16 in a peripheralportion of the array was 23 μm (the maximum value). The distribution ofdiameters of bottoms of the plated layers in central and peripheralportions of the array was about 10%.

Also in the mold of this embodiment, the distribution or variation ofthe semispherical or semicylindrical plated layers is decreased by theformation of the electroless plated layers on the electroplated layer.Further, since the gold electroless plated layer 16 containingphosphorus is formed on the surface, the corrosion resistance of thesurface of the mold is improved.

While the present invention has been described with respect to what arepresently considered to be the preferred embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments. The present invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A microstructure array comprising: a substrate having anelectrically-conductive portion; an insulating mask covering theelectrically conductive portion in which a plurality of openings arearranged to expose the electrically-conductive portion; and an array ofa microstructure each constituted by a first layer and a second layer,in which the first layer is disposed in each of the openings and on theinsulating mask and the second layer is laminated on the first layerwith a uniform thickness, wherein the first layer has a semisphere or asemicylinder shape and the second layer has a semisphere or semicylindershape according to the shape of the first layer, and wherein a thicknessof the second layer is uniform over the entire array such that adistribution of a bottom diameter of the microstructure in themicrostructure array is decreased from the distribution of the bottomdiameter of the semisphere or semicylinder of the first layer.
 2. Amicrostructure array according to claim 1, wherein each of the openingshas a circular shape and the microstructure is a semisphericalmicrostructure.
 3. A microstructure array according to claim 1, whereineach of the openings has an elongated stripe shape and themicrostructure is a semicylindrical microstructure.
 4. A microstructurearray according to claim 1, wherein microstructures of the array areconnected at their skirt portions.
 5. A microstructure array accordingto claim 1, wherein the distribution of the bottom diameter of thesecond layer between the central portion and the peripheral portion ofthe second layer is less than 20%.
 6. A microstructure array accordingto claim 1, wherein a third layer is further laminated on the secondlayer with a uniform thickness and a distribution of a bottom diameterbetween the central portion and the peripheral portion of themicrostructure including the third layer is smaller than thedistribution of the bottom diameter of the first layer.
 7. Amicrostructure array according to claim 1, wherein a bottom diameter ofthe microstructure in a central portion is smaller than a bottomdiameter of the microstructure in a peripheral portion.
 8. Themicrostructure array according to claim 7, wherein the microstructurearray is a mold for forming a microlens array, a lenticular lens or aflyeye lens.